A variety of multi-channel optical communications modules exist for simultaneously transmitting and/or receiving multiple optical data signals over multiple respective optical data channels. A Multi-channel optical transmitter module, as that term is used herein, denotes an optical communications module having multiple optical transmit channels for simultaneously transmitting multiple optical data signals over respective optical waveguides (e.g., optical fibers). A Multi-channel optical receiver module, as that term is used herein, denotes an optical communications module having multiple optical receive channels for simultaneously receiving multiple respective optical data signals over respective optical waveguides. A Multi-channel optical transceiver module, as that term is used herein, denotes an optical communications module having multiple optical transmit channels and multiple optical receive channels for simultaneously transmitting and receiving multiple optical data signals over respective transmit and receive optical waveguides. A bidirectional (BiDi) multi-channel optical transceiver module, as that term is used herein, denotes an optical communications module having multiple BiDi channels for simultaneously transmitting and receiving optical data signals over each BiDi channel.
For each of these different types of parallel optical communications modules, a variety of designs and configurations exist. A typical electrical subassembly (ESA) layout for a multi-channel optical communications module includes a circuit board, such as a printed circuit board (PCB), and various electrical components and optoelectronic components (i.e., laser diodes and/or photodiodes) mounted on the circuit board. A typical ESA layout for a multi-channel optical transmitter module includes a PCB, a controller IC chip mounted on the PCB, a multi-channel transmitter IC chip (i.e., a laser diode array driver IC chip) mounted on the PCB and a laser diode array chip mounted on the PCB. In some cases, an array of laser diode chips, sometimes referred to as laser diode singlets, are used instead of a single chip have an array of laser diodes integrated in it. Various other passive electrical components such as capacitors, resistors and inductors are typically also mounted on the PCB.
The multi-channel transmitter IC chip interfaces on one of its sides with the controller IC chip and on the opposite side with the laser diode array chip. The side of the multi-channel transmitter IC chip that interfaces with the controller IC chip can be thought of as the electrical interface side of the multi-channel transmitter IC chip. The side of the multi-channel transmitter IC chip that interfaces with the laser diode array chip can be thought of as the optical interface side of the multi-channel transmitter IC chip.
A typical ESA layout for a multi-channel optical receiver module is similar to that of the multi-channel optical transmitter module except that the multi-channel transmitter IC chip is replaced by a multi-channel receiver IC chip. A typical ESA layout for a multi-channel optical transceiver module is also similar, but has both a multi-channel transmitter IC chip and a multi-channel receiver IC chip and both a laser diode array chip and a photodiode array chip. It is also known to integrate the functionality of the multi-channel transmitter IC chip and of the multi-channel receiver IC chip into a single IC chip.
Demands for higher-bandwidth multi-channel optical communications modules have led to efforts to design and manufacture multi-channel optical communications modules that have greater numbers of channels, i.e., higher channel density. Higher channel density requirements make it more challenging to meet the high-performance design specifications required by the systems in which they are employed. High-speed multi-channel transmitter and/or receiver IC chips are often required to be monolithically integrated on a single IC chip. In such cases, the density of the channels and of the signals they carry is very high due to space limitations of the IC package. In order to meet demands for both increased signal speed and channel density, the IC layout becomes even more critical.
Traditional multi-channel transmitter and receiver IC chip layouts replicate channels next to each other, i.e., side by side. To simply replicate channels side by side in the layout may not meet the performance and feasibility requirements of high signal speed and high channel density applications. The design challenges for achieving high speed signals and high channel density include, for example, meeting spatial constraints, limiting or preventing channel-to-channel crosstalk, providing electromagnetic interference (EMI) shielding, meeting power consumption constraints, meeting heat dissipation requirements, meeting reliability expectations, and achieving good signal integrity.
The high speed multi-channel transmitter and receiver IC chip layout is also required to meet certain mechanical specifications. The channel-to-channel pitch should be very small on the optical interface side to match the pitch of the optoelectronic devices of the laser diode or photodiode array chips. The channel-to-channel pitch should remain constant over the entire lengths of the channels to ensure good signal integrity. However, on the electrical interface side, the electrical contacts usually need to have a greater pitch than the channel-to-channel pitch in order to meet the greater trace-to-trace pitch of the PCB. To accommodate the greater pitch between the electrical contacts, the electrical signal pathways that connect the channels with the electrical contacts can be fanned out, but this tends to compromise signal integrity to some extent. This fan out solution is more difficult to use for greater numbers of channels due to constraints on the IC package size.
The advantage of the traditional layout is that each channel is almost identical, which makes integration of the layout easier to achieve. However, side-by-side layout of the channels leads to some disadvantages. For high speed signals, the electrical cross talk between adjacent channels has a strong relationship with the distance between the signals. Small channel-to-channel distances introduce high levels of cross talk that severely degrade the performance of the circuits. In addition, because the channel circuit elements are high speed circuit elements, they usually consume a large amount of power. The concentration of power consumption in the channels creates a large amount of heat, which can cause the circuit elements to have poor reliability unless expensive additional heat dissipation measures are taken. Moreover, if circuit elements of the channels are so large that the channel-to-channel pitch cannot match the pitch of the optoelectronic elements of the optoelectronic array chip, a traditional layout may not be achievable.
A need exists for a layout for high speed multi-channel transmitter and receiver IC chips that ensures that the channel-to-channel pitch on the optical interface side of the chip matches the pitch of the optoelectronic elements of the optoelectronic array chips in order to achieve high channel density and good performance. A need also exists for a layout that achieves this goal while also ensuring that mechanical, thermal and electrical constraints of the transmitter or receiver IC package and of the PCB are met. A further need exists for a layout that achieves these goals while also reducing electrical cross talk and improving signal integrity.